Exactly this. In every one of the many states where I’ve lived, the main route to becoming a superdelegate was hard work for the local and state party. Often superdelegates are members of state/local government unions (especially teachers) because they understand the impact of local politics on their jobs so get strongly involved in local politics.
Opening up the primaries is a double-edged sword. In many states the active base is on the left of the broader Democratic electorate. I certainly think it should be easier to register, and NYC’s 6 month rule is way too excessive, but the party should focus their effort on getting people registered earlier.
Caucuses don’t limit the franchise, except in the sense that you actually have to take the time to go and participate, and care enough about your candidate that you have to be ready to defend him or her in a discussion with your neighbor. I’ve voted for president in 3 primary states and 3 caucus states (including Iowa), and wish all the states would move to caucuses.